During rapid switching of the gradient coils of magnetic resonance tomographs (MRT) sounds occur which can be very loud (>100 dB). The sound pressure level can rise within a few milliseconds from background noise to the maximum sound pressure level so that a real knocking sound occurs. Such knocking sounds can occur in examinations of vertebrates with magnetic resonance tomography and spectroscopy and in examinations with transcranial magnetic stimulation. These sounds cannot just damage the hearing but can also alarm the person being examined and are very unpleasant for them.
The current widely-used methods attempt to reduce the maximum sound pressure level and do this by constructional sound deadening methods or via a smaller gradient load. Another way is to reduce the sound pressure level at the ear, for example with headphones or ear plugs. Methods for sound extinction in the vicinity of the ear using interference are hardly ever implemented on account of the strong magnetic fields and the restricted space available. Headphones or earplugs also have only a very limited protective function since the loud knocking sounds can be transmitted not only via the auditory canal but also via the cranial bone into the inner ear and can thus simply thus not be filtered out just like that. Constructional sound deadening methods such as a heavier encapsulation of the coils and leads have only proved effective to a limited extent and reducing the sound by imposing less of a load on the gradient coils results in lower quality imaging.